[PATCH 1/3] sched: fix exit_mm vs membarrier (v4)

[PATCH 1/3] sched: fix exit_mm vs membarrier (v4)

[Mathieu Desnoyers]

exit_mm should issue memory barriers after user-space memory accesses,
before clearing current->mm, to order user-space memory accesses
performed prior to exit_mm before clearing tsk->mm, which has the
effect of skipping the membarrier private expedited IPIs.

exit_mm should also update the runqueue's membarrier_state so
membarrier global expedited IPIs are not sent when they are not
needed.

The membarrier system call can be issued concurrently with do_exit
if we have thread groups created with CLONE_VM but not CLONE_THREAD.

Here is the scenario I have in mind:

Two thread groups are created, A and B. Thread group B is created by
issuing clone from group A with flag CLONE_VM set, but not CLONE_THREAD.
Let's assume we have a single thread within each thread group (Thread A
and Thread B).

The AFAIU we can have:

Userspace variables:

int x = 0, y = 0;

CPU 0                   CPU 1
Thread A                Thread B
(in thread group A)     (in thread group B)

x = 1
barrier()
y = 1
exit()
exit_mm()
current->mm = NULL;
                        r1 = load y
                        membarrier()
                          skips CPU 0 (no IPI) because its current mm is NULL
                        r2 = load x
                        BUG_ON(r1 == 1 && r2 == 0)

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-mm@kvack.org
---
Changes since v1:
- Use smp_mb__after_spinlock rather than smp_mb.
- Document race scenario in commit message.

Changes since v2:
- Introduce membarrier_update_current_mm,
- Use membarrier_update_current_mm to update rq's membarrier_state from
  exit_mm.

Changes since v3:
- Disable interrupts around call to membarrier_update_current_mm, which
  is required to access the runqueue's fields.
---
 include/linux/sched/mm.h  |  5 +++++
 kernel/exit.c             | 16 +++++++++++++++-
 kernel/sched/membarrier.c | 12 ++++++++++++
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index f889e332912f..5dd7f56baaba 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -370,6 +370,8 @@ static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
 
 extern void membarrier_exec_mmap(struct mm_struct *mm);
 
+extern void membarrier_update_current_mm(struct mm_struct *next_mm);
+
 #else
 #ifdef CONFIG_ARCH_HAS_MEMBARRIER_CALLBACKS
 static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
@@ -384,6 +386,9 @@ static inline void membarrier_exec_mmap(struct mm_struct *mm)
 static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
 {
 }
+static inline void membarrier_update_current_mm(struct mm_struct *next_mm)
+{
+}
 #endif
 
 #endif /* _LINUX_SCHED_MM_H */
diff --git a/kernel/exit.c b/kernel/exit.c
index 733e80f334e7..18ca74c07085 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -475,10 +475,24 @@ static void exit_mm(void)
 	BUG_ON(mm != current->active_mm);
 	/* more a memory barrier than a real lock */
 	task_lock(current);
+	/*
+	 * When a thread stops operating on an address space, the loop
+	 * in membarrier_private_expedited() may not observe that
+	 * tsk->mm, and the loop in membarrier_global_expedited() may
+	 * not observe a MEMBARRIER_STATE_GLOBAL_EXPEDITED
+	 * rq->membarrier_state, so those would not issue an IPI.
+	 * Membarrier requires a memory barrier after accessing
+	 * user-space memory, before clearing tsk->mm or the
+	 * rq->membarrier_state.
+	 */
+	smp_mb__after_spinlock();
+	local_irq_disable();
 	current->mm = NULL;
-	mmap_read_unlock(mm);
+	membarrier_update_current_mm(NULL);
 	enter_lazy_tlb(mm, current);
+	local_irq_enable();
 	task_unlock(current);
+	mmap_read_unlock(mm);
 	mm_update_next_owner(mm);
 	mmput(mm);
 	if (test_thread_flag(TIF_MEMDIE))
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 168479a7d61b..8bc8b8a888b7 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -63,6 +63,18 @@ void membarrier_exec_mmap(struct mm_struct *mm)
 	this_cpu_write(runqueues.membarrier_state, 0);
 }
 
+void membarrier_update_current_mm(struct mm_struct *next_mm)
+{
+	struct rq *rq = this_rq();
+	int membarrier_state = 0;
+
+	if (next_mm)
+		membarrier_state = atomic_read(&next_mm->membarrier_state);
+	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
+		return;
+	WRITE_ONCE(rq->membarrier_state, membarrier_state);
+}
+
 static int membarrier_global_expedited(void)
 {
 	int cpu;
-- 
2.17.1

Re: [PATCH 1/3] sched: fix exit_mm vs membarrier (v4)

[Peter Zijlstra]

On Tue, Oct 20, 2020 at 09:47:13AM -0400, Mathieu Desnoyers wrote:
> +void membarrier_update_current_mm(struct mm_struct *next_mm)
> +{
> +	struct rq *rq = this_rq();
> +	int membarrier_state = 0;
> +
> +	if (next_mm)
> +		membarrier_state = atomic_read(&next_mm->membarrier_state);
> +	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
> +		return;
> +	WRITE_ONCE(rq->membarrier_state, membarrier_state);
> +}

This is suspisioucly similar to membarrier_switch_mm().

Would something like so make sense?

---
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -206,14 +206,7 @@ void membarrier_exec_mmap(struct mm_stru
 
 void membarrier_update_current_mm(struct mm_struct *next_mm)
 {
-	struct rq *rq = this_rq();
-	int membarrier_state = 0;
-
-	if (next_mm)
-		membarrier_state = atomic_read(&next_mm->membarrier_state);
-	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
-		return;
-	WRITE_ONCE(rq->membarrier_state, membarrier_state);
+	membarrier_switch_mm(this_rq(), NULL, next_mm);
 }
 
 static int membarrier_global_expedited(void)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d2621155393c..3d589c2ffd28 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2645,12 +2645,14 @@ static inline void membarrier_switch_mm(struct rq *rq,
 					struct mm_struct *prev_mm,
 					struct mm_struct *next_mm)
 {
-	int membarrier_state;
+	int membarrier_state = 0;
 
 	if (prev_mm == next_mm)
 		return;
 
-	membarrier_state = atomic_read(&next_mm->membarrier_state);
+	if (next_mm)
+		membarrier_state = atomic_read(&next_mm->membarrier_state);
+
 	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
 		return;
 

Re: [PATCH 1/3] sched: fix exit_mm vs membarrier (v4)

[Mathieu Desnoyers]

----- On Oct 20, 2020, at 10:36 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Tue, Oct 20, 2020 at 09:47:13AM -0400, Mathieu Desnoyers wrote:
>> +void membarrier_update_current_mm(struct mm_struct *next_mm)
>> +{
>> +	struct rq *rq = this_rq();
>> +	int membarrier_state = 0;
>> +
>> +	if (next_mm)
>> +		membarrier_state = atomic_read(&next_mm->membarrier_state);
>> +	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
>> +		return;
>> +	WRITE_ONCE(rq->membarrier_state, membarrier_state);
>> +}
> 
> This is suspisioucly similar to membarrier_switch_mm().
> 
> Would something like so make sense?

Very much yes. Do you want me to re-send the series, or you
want to fold this in as you merge it ?

Thanks,

Mathieu

> 
> ---
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -206,14 +206,7 @@ void membarrier_exec_mmap(struct mm_stru
> 
> void membarrier_update_current_mm(struct mm_struct *next_mm)
> {
> -	struct rq *rq = this_rq();
> -	int membarrier_state = 0;
> -
> -	if (next_mm)
> -		membarrier_state = atomic_read(&next_mm->membarrier_state);
> -	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
> -		return;
> -	WRITE_ONCE(rq->membarrier_state, membarrier_state);
> +	membarrier_switch_mm(this_rq(), NULL, next_mm);
> }
> 
> static int membarrier_global_expedited(void)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index d2621155393c..3d589c2ffd28 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2645,12 +2645,14 @@ static inline void membarrier_switch_mm(struct rq *rq,
> 					struct mm_struct *prev_mm,
> 					struct mm_struct *next_mm)
> {
> -	int membarrier_state;
> +	int membarrier_state = 0;
> 
> 	if (prev_mm == next_mm)
> 		return;
> 
> -	membarrier_state = atomic_read(&next_mm->membarrier_state);
> +	if (next_mm)
> +		membarrier_state = atomic_read(&next_mm->membarrier_state);
> +
> 	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
>  		return;

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH 1/3] sched: fix exit_mm vs membarrier (v4)

[Boqun Feng]

Hi,

On Tue, Oct 20, 2020 at 10:59:58AM -0400, Mathieu Desnoyers wrote:
> ----- On Oct 20, 2020, at 10:36 AM, Peter Zijlstra peterz@infradead.org wrote:
> 
> > On Tue, Oct 20, 2020 at 09:47:13AM -0400, Mathieu Desnoyers wrote:
> >> +void membarrier_update_current_mm(struct mm_struct *next_mm)
> >> +{
> >> +	struct rq *rq = this_rq();
> >> +	int membarrier_state = 0;
> >> +
> >> +	if (next_mm)
> >> +		membarrier_state = atomic_read(&next_mm->membarrier_state);
> >> +	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
> >> +		return;
> >> +	WRITE_ONCE(rq->membarrier_state, membarrier_state);
> >> +}
> > 
> > This is suspisioucly similar to membarrier_switch_mm().
> > 
> > Would something like so make sense?
> 
> Very much yes. Do you want me to re-send the series, or you
> want to fold this in as you merge it ?
> 
> Thanks,
> 
> Mathieu
> 
> > 
> > ---
> > --- a/kernel/sched/membarrier.c
> > +++ b/kernel/sched/membarrier.c
> > @@ -206,14 +206,7 @@ void membarrier_exec_mmap(struct mm_stru
> > 
> > void membarrier_update_current_mm(struct mm_struct *next_mm)
> > {
> > -	struct rq *rq = this_rq();
> > -	int membarrier_state = 0;
> > -
> > -	if (next_mm)
> > -		membarrier_state = atomic_read(&next_mm->membarrier_state);
> > -	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
> > -		return;
> > -	WRITE_ONCE(rq->membarrier_state, membarrier_state);
> > +	membarrier_switch_mm(this_rq(), NULL, next_mm);
> > }
> > 
> > static int membarrier_global_expedited(void)
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index d2621155393c..3d589c2ffd28 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -2645,12 +2645,14 @@ static inline void membarrier_switch_mm(struct rq *rq,
> > 					struct mm_struct *prev_mm,
> > 					struct mm_struct *next_mm)
> > {
> > -	int membarrier_state;
> > +	int membarrier_state = 0;
> > 
> > 	if (prev_mm == next_mm)

Unless I'm missing something subtle, in exit_mm(),
membarrier_update_current_mm() is called with @next_mm == NULL, and
inside membarrier_update_current_mm(), membarrier_switch_mm() is called
wiht @prev_mm == NULL. As a result, the branch above is taken, so
membarrier_update_current_mm() becomes a nop. I think we should use the
previous value of current->mm as the @prev_mm, something like below
maybe?

void update_current_mm(struct mm_struct *next_mm)
{
	struct mm_struct *prev_mm;
	unsigned long flags;

	local_irq_save(flags);
	prev_mm = current->mm;
	current->mm = next_mm;
	membarrier_switch_mm(this_rq(), prev_mm, next_mm);
	local_irq_restore(flags);
}

, and replace all settings for "current->mm" in kernel with
update_current_mm().

Thoughts?

Regards,
Boqun

> > 		return;
> > 
> > -	membarrier_state = atomic_read(&next_mm->membarrier_state);
> > +	if (next_mm)
> > +		membarrier_state = atomic_read(&next_mm->membarrier_state);
> > +
> > 	if (READ_ONCE(rq->membarrier_state) == membarrier_state)
> >  		return;
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com